Computer systems typically comprise a CPU and supporting subsystems, such as a memory subsystem, a storage subsystem and a display subsystem. Moreover, while some computer systems are contained within a single host computer box and share a single power source, the vast majority of modern computer systems have at least one peripheral device that is separate from the host computer box and which derives power from an independent power supply. Typically, such a peripheral device communicates with the subsystems in the host computer via a communication bus. These communication buses comprise ground, power, control lines, data lines and so on that links the peripheral device with the host computer.
Because the host computer and the peripheral device are independently powered, they must be independently disconnected from their respective power sources. For example, when the computer system is shut down, it may be the case that the power supply that supplies power to the host computer box is powered down while certain peripheral devices such as printers and disk drives remain powered on. Whenever one of the power source to the host computer is powered down before the peripheral device is powered down. As a result, power may continue to flow from the peripheral device over the communication bus.
This continued power flow has caused problems in some computer systems. Where a host computer is disconnected first, power can flow back from the peripheral subsystem into the host computer and wreak havoc on the states of certain subsystems within the host computer. Potentially, the back power could place the host computer into a state so that it will not properly restart without taking remedial measures.
Accordingly, there is a need for a peripheral device that prevents back powering host computers.